The present invention relates to a discharge lamp and a lamp unit. In particular, the present invention relates to a discharge lamp and a lamp unit used as the light source of an image projection apparatus such as a liquid crystal projector or a digital micromirror device (DMD) projector.
In recent years, an image projection apparatus such as a liquid crystal projector or a projector using a DMD has been widely used as a system for realizing large-scale screen images. A high-pressure discharge lamp having a high intensity has been commonly and widely used in such an image projection apparatus. For the light source used in the image projection apparatus, light is required to be concentrated on an imaging device included in the optical system of the projector, so that in addition to high intensity, it is also necessary to achieve a light source close to a point light source. Therefore, a short arc ultra high pressure mercury lamp that is closer to a point light and has a high intensity has been noted widely as a promising light source.
Referring to FIG. 9, a conventional short arc ultra high pressure mercury lamp 1000 will be described. FIG. 9 is a schematic view of an ultra high pressure mercury lamp 1000. The lamp 1000 includes a substantially spherical luminous bulb 110 made of quartz glass, and a pair of sealing portions (seal portions) 120 and 120′ also made of quartz glass and connected to the luminous bulb 110.
A discharge space 115 is inside the luminous bulb 110. A mercury 118 in an amount of the enclosed mercury of, for example, 150 to 250 mg/cm3 as a luminous material, a rare gas (e.g., argon with several tens kpa) and a small amount of halogen are enclosed in the discharge space 115. A pair of tungsten electrodes (W electrode) 112 and 112′ are opposed with a certain distance D (e.g., about 1.5 mm) in the discharge space 115. Each of the W electrodes 112 and 112′ includes an electrode axis (W rod) 116 and a coil 114 wound around the head of the electrode axis 116. The coil 114 has a function to reduce the temperature at the head of the electrode.
The electrode axis 116 of the W electrode 112 is welded to a molybdenum foil (Mo foil) 124 in the sealing portion 120, and the W electrode 112 and the Mo foil 124 are electrically connected by a welded portion 117 where the electrode axis 116 and the Mo foil 124 are welded. The sealing portion 120 includes a glass portion 122 extending from the luminous bulb 110 and the Mo foil 124. The glass portion 122 and the Mo foil 124 are attached tightly so that the airtightness in the discharge space 115 in the luminous bulb 110 is maintained. In other words, the sealing portion 120 is sealed by attaching the Mo foil 124 and the glass portion 122 tightly for foil-sealing. The sealing portions 120 have a circular cross section, and the rectangular Mo foil 124 is disposed in the center of the inside of the sealing portion 120.
The Mo foil 124 of the sealing portion 120 includes an external lead (Mo rod) 130 made of molybdenum on the side opposite to the side on which the welded portion 117 is positioned. The Mo foil 124 and the external lead 130 are welded to each other so that the Mo foil 124 and the external lead 130 are electrically connected at a welded portion 132. The structures of the W electrode 112′ and sealing portion 120′ are the same as those of the W electrode 112 and sealing 120, so that description thereof will be omitted.
Next, the operational principle of the lamp 1000 will be described. When a start voltage is applied to the W electrodes 112 and 112′ via the external leads 130 and the Mo foils 124, discharge of argon (Ar) occurs. Then, this discharge raises the temperature in the discharge space 115 of the luminous bulb 110, and thus the mercury 118 is heated and evaporated. Thereafter, mercury atoms are excited and become luminous in the arc center between the W electrodes 112 and 112′. The higher the mercury vapor pressure of the lamp 1000 is, the higher the emission efficiency is, so that the higher mercury vapor pressure is suitable as a light source for an image projection apparatus. However, in view of the physical strength against pressure of the luminous bulb 110, the lamp 1000 is used at a mercury vapor pressure of 15 to 25 MPa.
As shown in FIG. 10, the lamp 1000 can be formed into a lamp unit 1200 in combination with a reflecting mirror 60. The lamp unit 1200 includes the discharge lamp 1000 and the reflecting mirror 60 for reflecting light emitted from the discharge lamp 1000, and the light emitted from the discharge lamp 1000 is reflected at the reflecting mirror 60 and emits in the emission direction 50. The reflecting mirror 60 has a front opening 60a on the side of the emission direction 50. A front glass (not shown)is to be attached at the front opening 60a for the purpose of preventing scattering at the time of lamp breakage.
A lead wire 65 is electrically connected to the external lead 130 of the sealing portion 120 positioned on the front opening 60a side. The lead wire 65 for external connection is formed of, for example, a Ni—Mn alloy, and extends from the junction 131 with the external lead 130 to the outside of the reflecting mirror 60 through an opening 62 for a lead wire so as to be electrically connected to an external circuit (e.g., ballast). A lamp base 55 is attached to the other sealing portion 120′ of the discharge lamp 1000, and the sealing portion 120′ is attached to the reflecting mirror 60.
To electrically connect the external lead 130 of the sealing portion 120 to the lead wire 65 for external connection, the first approach that one can come up with is to simply wind the lead wire 65 for external connection around the external lead 130. However, the approach of simply winding is not sufficient for electrical connection (electrical conductivity) between the lead wire 65 for external connection and the external lead 130 because the lead wire 65 and the external lead 130 are not welded. Therefore, it is possible that discharge occurs at the junction 131, and therefore it is not preferable to use this approach to join the lead wire 65 for external connection 130. Thus, the external lead 130 and the lead wire 65 for external connection in the lamp unit 1200 are joined by welding.
Molybdenum constituting the external lead 130 has the property of being recrystallized at high temperatures and becoming fragile, and therefore it is technically difficult for the external lead 130 and the lead wire 65 for external connection to be joined directly by welding. Therefore, the external lead 130 and the lead wire 65 for external connection are welded at a low temperature in the following manner, as shown in FIG. 11. First, a sleeve (cylinder) 140 made of Ni is placed in such a manner that the sleeve 140 is in contact with the outer circumference of the junction 131 of the external lead 130, and then the external lead 130 and the sleeve 140 are welded at a relatively low temperature. Then, the sleeve 140 and the lead wire 65 for external connection made of a Ni—Mn alloy are welded. Thus, it is possible to electrically connect the external lead 130 and the lead wire 65 for external connection while preventing the external lead 130 from being fragile.
However, the welding portion 142 between the sleeve 140 and the lead wire 65 for external connection is formed by point welding, so that the contact area is small (almost a point contact). Therefore, when stress is applied to the lead wire 65 for external connection, the lead wire 65 for external connection is easily dropped off from the junction 131. In particular, when assembling the lamp unit 1200, it is necessary to pass the lead wire 65 for external connection through the opening 62 for a lead wire of the reflecting mirror 60. Therefore, stress is easily applied to the lead wire 65 for external connection, and the lead wire 65 for external connection is often dropped off. Furthermore, the welded portion 144 between the external lead 130 and the sleeve 140 also is formed by point welding. Therefore, if stress is applied to the sleeve 140, the sleeve 140 may be moved, and the welded members may be detached so that the sleeve 140 may be dropped off. Therefore, in the conventional lamp unit 1200, the reliability in the connection between the external lead 130 and the lead wire 65 for external connection is not good.
In the past, the lamp lifetime was comparatively short, so that even if the reliability in the connection between the external lead 130 and the lead wire 65 for external connection is poor to some extent, this drawback alone rarely causes a big problem. However, nowadays when the lamp lifetime has been prolonged to, for example, 2000 hours or more because of improvement of production techniques or the like, it is important to improve the reliability in the connection between the external lead 130 and the lead wire 65 for external connection, and this problem of the connection reliability is expected to become serious.